Alternative copper electrolytes

Question: We have been copper and silver plating aluminum parts for a long-standing customer for years. Now a new series of articles has been announced for which we have to deposit between 200 and 500 µm of copper instead of the usual 20 µm. As the customer is known for time-critical orders that often have to be carried out overnight, our acidic copper electrolytes are already highly optimized. In our opinion, however, this is no longer feasible for the new series of articles. In our search for alternatives, we came across pyrophosphate and fluoroborate electrolytes, but the information on these is extremely scarce. Even our usual suppliers do not offer these and point out that they are outdated technologies. Can you help us further?

Answer: Unfortunately, we are very familiar withthis problem. In this specific case, the more up-to-date the sources are, the less information is available. In some cases, reference is only made to the fact that such electrolytes once existed. From a market perspective, it may make sense for specialist companies to no longer offer such exotic electrolytes, but there are plenty of examples in history where "outdated technologies" are the perfect solutions to modern problems. Nevertheless, you should check whether the - currently still theoretical - problems cannot also be solved using existing processes. Modern, acidic copper electrolytes can certainly be used for very fast and yet technically flawless coatings. The advantage of further optimizations would be that you would not need any new chemicals and would work with processes that you are already familiar with.

However, we would like to provide you with all the relevant information on the two types of electrolyte that we have below. This should enable you to start your own preliminary tests.

Pyrophosphate electrolytes

These electrolytes are produced from copper pyrophosphate, which is slightly soluble in water. However, it is very soluble when alkali diphosphate is added. To ensure that the electrolyte conducts well and the anodes are well dissolved, it is advantageous to have an excess of potassium pyrophosphate in the electrolyte.

Typical preparation for standard electrolyte:

• Copper pyrophosphate: 110 g/L
• Potassium pyrophosphate: 400 g/L
• Citric acid: 10 g/L
• Ammonia 25 %: 3 g/L
• Temperature: 50 - 70 °C
• Current density: 0.5 - 8 A/dm2
• Brighteners: N-methylpyrrolidone, malic acid, hydroboric acid, lactic acid, 2,5-dimercapto- 1,3,4-thiadiazole (DMcT)
• pH value: 8.5

Ascorbic acid can also be added as an antioxidant.

Copper pyrophosphate

Copper(II) pyrophosphate is a chemical compound consisting of copper ions and pyrophosphate ions. Its chemical formula is _Cu2P2O7, where the pyrophosphate ion (_P2O74-⁾ is made up of two phosphate groups linked by an oxygen bridge. The compound typically has a bluish or greenish color, which is characteristic of many copper compounds.

Potassium pyrophosphate

Potassium pyrophosphate, also known as diphosphoric acid tetrapotassium salt, is a water-soluble, crystalline salt with the molecular formula _K4P2O7. It belongs to the group of pyrophosphates, which are characterized by the combination of two phosphate groups. In aqueous solution, it shows an alkaline reaction as it acts as a weak base and can absorb protons. It has complexing properties, which makes it particularly useful for electroplating applications. Due to its ability to hold metal ions in solution, it is also used as a sequestering agent in detergents and in the food industry. At higher temperatures it can break down into simpler phosphates.

In electroplating, pyrophosphate is also used in alloy electrolytes, among other things. These include copper-tin, nickel-cobalt, tin-cobalt and nickel-iron electrolytes.

Anodes

Soluble copper anodes (e.g. electrolytic copper) are predominantly used. In order not to fall below an efficiency of ≈100 %, an anodic current density of less than 2 ^A/dm2 is recommended. In general, the maximum anodic current density depends on the alkalinity of the electrolyte.

However, insoluble anodes are also used:

• Platinum or iridium dioxide-coated titanium anodes
• Graphite anodes as a cheaper alternative, but with limited durability.

Modifications

As is almost always the case in electroplating technology, pyrophosphate electrolytes also have numerous modifications with different compositions and bath parameters. However, as these electrolytes are generally used rather rarely, the modifications are also rather less popular. Of practical interest is a recipe for the direct electroplating of aluminium, which became somewhat more widespread in the 1960s:

• Copper sulphate: 30 - 35 g/L
• Sodium pyrophosphate: 120 - 145 g/L
• Temperature: 50 - 70 °C
• Current density: 1.2 - 1.7 ^A/dm2
• pH value: 7.5 - 8.0

The cathodic current density is ≈92 %. The coating has good adhesive strength on aluminum and is at the level of copper-plated aluminum surfaces that were previously coated with zincate.

Fluoroborate electrolytes

Acidic copper fluoroborate electrolytes are used to deposit very thick copper layers with high current densities. They are suitable for current densities of up to 40 A/dm2 - a parameter that is particularly important in wire copper plating and electroforming. Thanks to the high solubility of the complex anion (BF4-), higher copper ion concentrations can also be achieved in the electrolyte.

As the copper ions in the electrolyte are present in double valence, certain ferrous materials must first be pre-copper-plated with cyanide. The copper layers deposited in this way are characterized by a fine crystalline structure and a smoother surface than layers that originate from sulphuric acid electrolytes. However, the distribution of the layer thicknesses is less homogeneous, which is why this electrolyte should preferably be used for low-profile components.

Due to the high cost of copper fluoroborate concentrate, the corrosive effect of boric hydrofluoric acid on the environment and the special requirements for waste water treatment, the use of a copper fluoroborate electrolyte must be carefully considered.

Typical approach

• Copper fluoroborate: 336 g/L
• Tetrafluoroboric acid: 2 g/L
• Boric acid: 20 g/L
• Temperature: 25 - 50 °C
• Current density: 12 - 15 ^A/dm2
• pH value: 0.5 - 0.7
• Current yield: 98%

Electrolyte properties

Copper can be deposited just as brightly from fluoroborate electrolytes as from sulphate baths.

With copper fluoroborate solutions containing 100 g/L copper, perfect deposits can be achieved on medium-profiled workpieces at 28 °C and a current density of approx. 20 A/dm2. Current densities of 50 to 60 A/dm2 can even be achieved on rotary rollers where electrolyte movement is generated by stirring or blowing in air. Maintenance of the electrolyte is limited to monitoring the copper content, density and pH value.

The anode solubility proves to be good, while sludge formation is lower than in the copper sulphate electrolyte. Lead impurities should be avoided; iron contents of up to 3 g/L have no influence on the operation.

Organic impurities lead to brittle and discolored precipitates and must therefore be removed by activated carbon filtration. The addition of small amounts of copper sulphate and a commercially available brightener to the fluoroborate electrolyte results in shiny and even deposits.

In addition to pure fluoroborate electrolytes, mixed solutions with sulphuric acid are also used. These have the advantage that lead impurities are rendered harmless immediately, as lead precipitates as lead sulphate.

Due to the very rapid deposition, these electrolytes are used in particular for pipes, wires, strips and occasionally for printed circuit boards. However, the disadvantages include the significantly higher costs and the corrosive effect on the system.

Anodes

Tempered rolled or electrolytic copper is used as the anode material. The anodes should be arranged in a 1:1 ratio of anode to cathode. Wrapped anodes develop a passive film more quickly; in addition, very fine copper particles are continuously released from the anodes during electrolysis and migrate to the cathode, where they are also deposited and lead to rough deposits.

Alkaline cyanide-free electrolytes, which are mainly used for steel, aluminum and zinc die casting, are also suitable for pre-copper plating or as an alternative. Further information can be found in the specified course.